Method and apparatus for heating metal strip



July 4, 1967 HANS-HEINZ MULLER ETAL 3,329,591

METHOD AND APPARATUS FOR HEATING METAL STRIP 5 Sheets-Sheet 1 Filed Sept. 21, 1964 y 4, 1967 HANS-HEINZ MULLER ETAL 3,329,591

METHOD AND APPARATUS FOR HEATING METAL STRIP Filed Sept. 21, 1964 5 Sheets-Sheet 2 Fig.2

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METHOD AND APPARATUS FOR HEATING METAL STRIP Filed Sept. 21, 1964 v 3 Sheets-Sheet 3 2 }nvenlors United States Patent 3,329,591 METHOD AND APPARATUS FOR HEATING METAL STRIP Hans-Heinz Miiller, 11 Sieperstrasse, Remscheid, Germany, and Herbert Geisel, 16 Olper Hohe, Remscheid- Luttringhausen, Germany Filed Sept. 21, 1964, Ser. No. 397,780

Claims priority, application Germany, Sept. 21, 1963,

D 42,535 14 Claims. (Cl. 204-37) The invention relates to a method of and apparatus for inductively heating the edges of travelling metal strip. As known it is a matter of particular difliculty to heat a thin metal strip uniformly across the whole of its width. For drying purposes or for evenly fusing a coating on to the strip a uniform heating effect is essential. Such coatings may be lacquers or sprayed films of plastic which require consolidation and drying. The fact that the edges of metal strip reach the required temperatures later than the strip center is a particular nuisance in the process of fusing an electrolytically deposited film of tin on to a metal strip.

To this end the strip may be heated in various ways, for instance in a gas or electrically heated furnace. Alternatively, heating may be accomplished by the direct passage of current through the strip or by inductive heating. In any such case the temperature rise at the strip edges lags behind that in the strip centre.

For illustrating the difficulties which the invention seeks to overcome, the method of inductively heating metal strip may here be described, particularly when this is performed for the purpose of fusing coatings, such as electrolytically deposited films of tin, on to the strip. The conventional procedure consists in taking the strip over rollers and passing the same through several high and medium frequency coils arranged in consecutive groups. Usually the strip is carried up and down over the rollers.

In order that the present invention may be explained and clearly understood reference is hereinafter made to the accompanying drawings, in which FIGURE 1 is a diagrammatic lay-out of a strip-heating apparatus adapted according to the present invention; FIGURE 2 is a fragmentary view which will be hereinafter explained, FIG- URE 3 is a face view of one form and arrangement of inductor according to the invention and FIGURE 4 is a view of another such form and arrangement.

Referring to FIGURE 1, the thin sheet iron strip, as indicated at 1, runs through a tank 2 containing a hot liquid. From the tank the strip is taken upwards to a deflecting roller 3 whence it descends through a group of coils 4 which in the illustrated example comprises nine coils. The strip then runs on to the further deflecting roller 3' and ascends to a roller 3" through another group of coils6 comprising ten coils. Groups 4 and 6 preheat the strip, whereas it is raised to the fusing temperature of the tin as it descends through the final group 7 comprising six coils.

It is now desired that the fusing temperature should be reached uniformly across the full width of the strip before the strip is submerged below the level 8 of a quenching fluid in a tank 9 in which the strip is again deflected at 10.

It will be understood from the schematic drawing in FIGURE 2 that the conventional arrangement does not achieve this satisfactorily. In the last three coils 12, 13 and 14 of group 7 the temperature distribution across the strip marked 11 is roughly as indicated'by line 15. In other Words, -a uniform temperature slightly exceeding 230 is reached across a substantial portion of the width of the strip, as required for ensuring that the film of tin will fuse on to the strip. In practice this line is clearly visible because the strip has a mat appearance before the tin melts, whereas the portion where the tin has already fused has a lustre which it retains even after it is quenched.

3,329,591 Patented July 4, 1967 However, in the marginal regions 16 along the strip edges this temperature is not reached, or the tin does not fuse until a later time. The fusion boundary line is therefore arched. Moreover, this line fails to straighten out before the strip is plunged into the quenching fluid at 8.

By increasing the power or by varying the speed of travel of the strip the desired temperature distribution cannot be achieved. The only result is to raise the temperature in region 15 beyond the fusing temperature of the tin. This leads to the formation of a strongly developed intermediate iron-tin layer which is brittle, and which therefore causes trouble during the further processing of the strip. The acceptance of a lacquer by the strip is also adversely affected. Furthermore, the downward branches 17 of the boundary line are lengthened, indicating that the marginal zones in which the tin has not been properly fused have actually become larger still. At the same time care must be taken to ensure that the fusion boundary line does not shift too far upwards because otherwise fusion may already be initiated in the region of roller 3", causing the surface to be marked.

When other heating methods are used similar difficulties may arise and these can likewise not be overcome by raising the heating power of the heating means or by changing the speed of travel of the strip. More particularly, in the process of inductive heating a higher coupling factor in the region of the strip edges or a change of the effective surfaces of the coils in this region does not achieve the desired result. It is also a remarkable fact that the temperature distribution cannot be made more uniform by providing supplementary coils which embrace only the strip edges.

For solving the contemplated problem the present invention proposes to induce current filaments in the strip by providing supplementary inductive heating, said current filaments flowing lengthwise of the strip along the margins of strip and forming closed circuits across the width of the strip. As a result of this step each section of the strip edge remains under the influence of the induced current for a considerable period, whereas the central region of the strip is continuously withdrawn from the effect of the current flowing across the strip in accordance with the speed at which the strip travels. In this way the strip edges are automatically considerably heated, whereas the temperature rise in the central region of the strip is practically negligible. The length of the descending branches 17 of the fusion boundary line (cf. FIGURE 2) can thus be successfully reduced to zero and the same temperature achieved in the regions 16 as at 15. The resultant fusion boundary line is therefore made to extend across the entire strip width and the film of tin is uniformly fused.

For performing the method the invention proposes to use an inductor in which heating conductors forming loops above the surface of the strip have sections extending lengthwise in the neighbourhood of the strip edges and sections connecting the same and extending across the width of the strip. An inductor thus contrived induces current filaments in the strip which substantially reflect its shape, the currents flowing along the strip edges for a considerable distance. Such an inductor may be duplicated, that is to say an inductor through which the current flows in the same direction may be provided on each side of the strip.

In a particularly useful arrangement an inductor is arranged to embrace the strip and comprises lengthwise sections of heating conductor extending in the neighbourhood of the strip edges and interconnected by sections extending across the width of the strip alternately above and below the same.

The inductors which form closed loops above the strip surface as well as the inductors which embrace the strip may have a considerable diversity of shapes. The most convenient form is an inductor of rectangular shape comprising sections extending across the strip of a length roughly corresponding to or exceeding the width of the strip. Alternatively, inductors of trapezoidal or elliptical shape might be used.

In the latter case the major axis of the ellipse should extend in the lengthwise direction of the strip and the minor axis should preferably be equal to the width of the strip.

For clarifying the invention in greater detail preferred embodiments of inductors according to the invention are shown in FIGURES 3 and 4.

FIGURE 3 shows a rectangular inductor seen in plan from above the strip 1. The inductor forms a rectangular loop, the sides 19 of the rectangle extending parallel to the strip edges. The strip travels through the inductor interior in such manner that on the entry side the side 20 of the rectangle extending across the width of the strip faces the underside of the strip, whereas the other side 21 of the rectangle crossing the strip faces the top. Naturally the position of the two sides 20 and 21 in relation to the strip could be exchanged.

The inductor is supplied with current from 22. The pattern of the induced current is indicated by the discontinuous lines 23-24. Owing to the proximity effect the induced current filaments at the strip edges, as indicated at 23, are squeezed closer together and thus bring about a more intense heating effect in the relative strip sections, so that by suitably selecting the dimensions the temperature drop at 16, FIGURE 2, can be compensated. Within a relatively wide tolerational range the difference between the strip width and the clear width of the inductor is immaterial. It is therefore possible to heat strips of different widths with one and the same inductor provided the difference between the respective dimensions is not excessive. Moreover, the running strip may migrate to and fro in the crosswise direction without any adverse effect upon the process of fusion. Alternatively, the width of the inductor may be arranged to be adjustably variable, for instance by constructing the sides 20-21 of the rectangle of telescopically variable sections. The same arrangement may also be provided for the longitudinal sides 19. By thus varying the size of the free interior space the inductor can be adapted to a very wide range of conditions, such as strip width, speed of travel, fusing and drying requirements.

Hence, an inductor according to the invention, for instance in combination with apparatus such as that shown in FIGURE 2, represents a simple means for fusing coatings, such as films of tin, on to metal strip, drying coatings of lacquer, consolidating plastic coatings and so forth, because the effect of the inductor is uniform across the entire width of the strip.

If an inductor of such a kind is used in conjunction with an arrangement, such as that illustratively shown in FIGURE 1, the inductor 18 is located in the direction of travel of the strip behind roller 3 and thus directly precedes the group of coils 7. Alternatively the proposed inductor could be arranged to precede roller 3", as indicated in dotted lines at 18.

From the description of the inductor shown in FIG- URE 3 it will be understood that the inductor need not necessarily always be of rectangular shape and that trapezoidal, oval or other shapes are quite feasible. The essential requirement is that the inductor should induce longitudinal current filaments in the neighbourhood of the strip edges. Since the strip moves, a point on a strip edge will remain under the influence of a current filament 23 for as long a time as the strip needs to travel the length of the side section 19 of the inductor. A point on the strip in the centre thereof travels directly across the current filament at 24. A heating effect in this region is therefore substantially absent.

If for reasons of space coil 18 or 18' cannot be arranged as indicated in FIGURE 1, the supplementary coil may be arranged as indicated in FIGURE 4. The perspective representation of the upper deflecting roller 3 indicates that the inductor 25 is located above the roller and adapted to the curvature of the strip 11, section 26 being above and section 27 below the strip. The arrangement according to FIGURE 4 has the advantage of needing little space along the free length of the strip 11. This latter inductor may also be constructed to be of adjustable length and width.

What we claim is:

1. A method of raising the temperature of a generally longitudinally moving metal strip to a higher, predetermined temperature, uniform across the width of the strip comprising: first heating the central portion of the strip to said predetermined temperature by first heating means; and subsequently heating the longitudinally extending marginal edges of the strip to said predetermined temperature by induction heating means separate from said first heating means, having current lines extending generally parallel to and adjacent said marginal edges and having current paths which close across the width of the strip, whereby each quantum of the marginal edges remains under the influence of the induced current for a considerable period, whereas each quantum of the central portion of the strip is negligibly influenced by the induced current.

2. The method according to claim 1, in which the strip is coated with a coating medium before the first heating step, and the coating is fused onto the metal strip during the first and said subsequent heating stages.

3. The method according to claim 2, in which a coating of tin is deposited on the metal strip before the first heating stage and the tin is fused onto the metal strip during the first and said subsequent heating stages.

4. Apparatus for raising the temperature of a generally longitudinally moving strip to a higher, predetermined temperature, uniform across the width of the strip comprising: means for conveying a metal strip generally longitudinally through a first and subsequent heating stages; first heating means for heating the central portion of the moving metal strip; and subsequent heating means for heating the longitudinal edges of the strip, said subsequent heating means including an inductor comprising: a heating conductor forming a loop arranged to lie adjacent one face of the strip said loop having sections arranged to extend adjacent the longitudinal edges of the strip and sections interconnecting the said longitudinally extending sections and arranged to extend across the width of the strip.

5. Apparatus according to claim 4, in which a second inductor substantially the duplicate of the first mentioned inductor is arranged to lie adjacent the opposite face of the strip from said one face, both inductors being arranged so that current flows in the same direction in each inductor.

6. Apparatus according to claim 4 in which said inductor is of rectangular shape in which the conductor sections arranged to extend across the strip have a length at least equal to the width of the strip.

7.' Apparatus as claimed in claim 4 in which the inductor is of trapezoidal shape.

8. Apparatus as claimed in claim 4 in which the inductor is of elliptical shape arranged so that the major axis extends in the longitudinal direction of the strip and the minor axis is substantially as long as the strip is wide.

9. Apparatus for raising the temperature of a generally longitudinally moving metal strip to a higher predetermined temperature, uniform across the width of the strip comprising: means for conveying a metal strip generally longitudinally through first and subsequent heating stages, first heating means for heating the central portion of the moving metal strip, and subsequent heating means consisting of an inductor arranged to embrace the strip and comprising first heating conductor sections arranged to extend adjacent the longitudinal edges of the strip and further conductor sections interconnecting said first sections and arranged to extend across the width of the strip, one of said further sections being located to lie adjacent one face of the strip and the other of said further sections being located to lie adjacent the other face of the strip.

10. Apparatus according to claim 9 in which said inductor is of rectangular shape in which the conductor sections arranged to extend across the strip have a length at least equal to the width of the strip.

11. Apparatus according to claim 9 in which the inductor is of trapezoidal shape.

12. Apparatus according to claim 9 in which the inductor is of elliptical shape arranged so that the major axis extends in the lengthwise direction of the strip and the minor axis is substantially as long as the strip is Wide.

13. Apparatus according to claim 9 further comprising a deflecting roller, and in which the said first heating conductor sections extending lengthwise of the strip are arched and located in the region of said roller and substantially conformed to the path of the strip as it travels over said roller.

14. Apparatus for raising the temperature of a generally longitudinally moving metal strip to a higher, predetermined temperature, uniform across the width of the and subsequent heating stages and a curved path segment first heating means for heating the central portion of the moving metal strip, and subsequent heating means consisting of an inductor comprising two arched heating conductor sections adapted to lie adjacent the longitudinal edge of the strip when travelling through said curved path segment and comprising heating conductor sections connecting said arched sections and adapted to lie one on one side and the other on the other side of said strip within said curved path segment.

References Cited UNITED STATES PATENTS 2,897,328 7/1959 Alf et a1. 21910L61 3,008,026 11/1961 Kennedy 21910.61 3,031,555 4/1962 Ross et al. 219-10.61

FOREIGN PATENTS 593,195 10/ 1947 Great Britain. 641,053 8/ 1950* Great Britain. 767,448 11/ 1952 Germany.

OTHER REFERENCES Fritz, German application 1,084,850 printed July 7, 1960 (K1 21 11 29/30).

JOHN H. MACK, Primary Examiner.

RICHARD M. WOOD, Examiner.

L. H. BENDER, W. VAN SISE, Assistant Examiners. 

1. A METHOD OF RAISING THE TEMPERATURE OF A GENERALLY LONGITUDINALLY MOVING METAL STRIP TO A HIGHER, PREDETERMINED TEMPERATURE, UNIFORM ACROSS THE WIDTH OF THE STRIP COMPRISING: FIRST HEATING THE CENTRAL PORTION OF THE STRIP TO SAID PREDETERMINED TEMPERATURE BY FIRST HEATING MEANS; AND SUBSEQUENTLY HEATING THE LONGITUDINALLY EXTENDING MARGINAL EDGES OF THE STRIP TO SAID PREDETERMINED TEMPERATURES BY INDUCTION HEATING MEANS SEPARATE FROM SAID FIRST HEATING MEANS, HAVING CURRENT LINES EXTENDING GENERALLY PARALLEL TO AND ADJACENT SAID MARGINAL EDGES AND HAVING CURRENT PATHS WHICH CLOSE ACROSS THE WIDTH OF THE STRIP, WHEREBY EACH QUANTUM OF THE MARGINAL EDGES 